Digital multimeters (DMMs) a re well known test equipment which are used to provide many electrical measurements, including ratio-metric resistance measurement. Prior DMMs used RMS converters to perform this measurement. But RMS converters have relatively poor accuracy. Recently DMMs have included delta-sigma ADCS followed by digital signal processing (DSP) circuitry to perform this measurement. This arrangement has an improved accuracy compared to DMMs imp lemented using RMS converters. In addition, this arrangement provides improved AC voltage measurement, and makes it possible to provide other desired measurements, such as combined DC and AC vo ltage measurement.
It is known that a delta-sigma converter with a relatively wide common mode range is required to perform ratio-metric resistance measurements. It is also known that common mode range may be traded off against dynamic range; that is, common mode input range may be extended by decreasing the dynamic range. Decreasing the dynamic range, however, decreases the performance of the digital multimeter.
A delta-sigma ADC consists of two parts, a delta-sigma codulator, followed by a digital filter and decimator. The delta-sigma modulator generates an oversampled pulse-width-modulated (PWM) digital signal having an average voltage corresponding to the voltage of the input analog signal. The delta-sigma modulator reconverts the output PWM digital signal to an analog reference signal which is then fed back to the input of the delta-sigma modulator. One form of a delta-sigma modulator includes one or more serially connected integrators which have been implemented using known switched capacitor (SC) techniques.
Prior SC delta-sigma modulators include an input switched capacitor coupled between an input terminal receiving an analog input signal and an integrator. Thus, the input signal is AC coupled to the integrator. In one prior art arrangement, the feedback signal is coupled to the input terminal side of the input switched capacitor, via a feedback switched capacitor. In another prior art arrangement, the feedback signal is coupled to the integrator side of the input switched capacitor, also using a feedback switched capacitor. The gain of the feedback reference signal is controlled by the ratio of the feedback capacitor and the input capacitor. In both of the prior art arrangements, the feedback reference signal is also AC coupled to the amplifier through the feedback capacitor, and/or through the input capacitor as well. This restricts the common mode range of the ADC to the range of the reference feedback voltage.
A delta-sigma modulator which provides increased common mode range, without requiring a concomitant decrease in dynamic range is desirable.
In accordance with principles of the present invention, a delta-sigma modulator includes an integrator which includes an integrating amplifier having an input terminal and an output terminal. A pole of a first single-pole-double-throw (SPDT) switch is coupled to a first electrode of an input capacitor, while a first throw of the first SPDT switch is coupled to an input signal source, and a second throw is coupled to a source of reference potential (ground). A pole of a second SPDT switch is coupled to a second electrode of the capacitor, while a first throw of the second SPDT switch is coupled to the input terminal of the integrating amplifier. A feedback signal generator has an input terminal coupled to the output terminal of the integrating amplifier, and an output terminal directly coupled to a second throw of the second SPDT switch.
A delta-sigma modulator according to the present invention applies the feedback reference signal to the input terminal of the integrator without passing through a capacitor. The reference voltage, thus, is DC coupled to the input of the integrator while, the common mode input signal is AC coupled to the input terminal of the integrator by the input switched capacitors. This results in the common mode voltage being limited only by the supply voltage, and not by the reference voltage as in prior art delta-sigma converters. The dynamic range of the delta-sigma converter, however, is not affected by this arrangement.